Accessor and Rails for a Data Storage Library

ABSTRACT

Data storage libraries as disclosed that provide for accessors that turn upon angled rails. In the library, a first rail and a second rail form an angle. The first and second rail each have a drive surface, but the drive surfaces are on different vertical planes. A switching mechanism of the accessor is then configured to move a drive mechanism between the different vertical planes of the drive surfaces. If the switching mechanism positions the drive mechanism adjacent to the drive surface of the first rail, then the drive mechanism engages the drive surface of the first rail to move the accessor along the first rail. If the switching mechanism switches the position of the drive mechanism adjacent to the drive surface of the second rail, then the drive mechanism engages the drive surface of the second rail to move the accessor along the second rail.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of data storage libraries and, inparticular, to improved accessors and rails for data storage libraries.

2. Statement of the Problem

Data storage libraries provide a cost effective way of storing largeamounts of data. Data storage libraries may use magnetic tape, magneticdisks, optical tape, optical disk, etc, as the storage media for data. Adata storage library that uses magnetic tape is referred to as a tapelibrary. A typical tape library includes a plurality of storage shelvesfor storing tape cartridges. The tape library may vary in size fromstoring a few tape cartridges to storing thousands of tape cartridges.The storage shelves comprise multiple columns and rows of storage slotsfor storing tape cartridges. The tape cartridges are readable andwriteable by one or more tape drives in the tape library. One or morerobotic accessors are used to transport tape cartridges between thestorage shelves and the tape drives. Accessors are also referred to asrobotic pickers, robotic arms, etc. The movements of the accessors arecontrolled by a library control unit.

To access a selected file stored in the tape library, a host computercontains information from which it can map a particular file to the tapecartridge on which the file is stored. The host computer transmits acommand to the tape drive. The tape drive then transmits the command tothe library control unit. Under control of the library control unit, anaccessor is operable to locate a particular tape cartridge on thestorage shelves, retrieve the tape cartridge from a storage shelf,transport the tape cartridge to a tape drive, and insert the tapecartridge into the tape drive. The tape drive then reads data from orwrites data to the magnetic tape of the tape cartridge. After use of thetape cartridge is finished, the accessor is operable to remove the tapecartridge from the tape drive, transport the tape cartridge to theappropriate storage shelf, and return the appropriate tape cartridge tothe storage shelf.

FIG. 1 illustrates a tape library 100 comprising a single storage frame102 in the prior art. A storage frame comprises a section or modulehaving a certain number of storage shelves having slots to store mediacartridges, such as tape cartridges. Storage frame 102 includes aplurality of storage shelves 104-105 having slots for storing tapecartridges. Storage shelves 104 are on a front wall of storage frame102, and storage shelves 105 are on a back wall of storage frame 102.Storage frame 102 also includes a plurality of tape drives 106 forreading data from or writing data to the tape cartridges stored in thestorage shelves 104-105. Storage frame 102 also includes an accessor108.

Accessor 108 is operable to transport tape cartridges between thestorage shelves 104-105 and the tape drives 106. Accessor 108 comprisesa gripper assembly 112 for gripping one or more tape cartridges andtransporting the tape cartridges between the storage shelves 104-105 andtape drives 106. The gripper assembly 112 is mounted to a vertical rail114 (also referred to as a Y-rail) and may be moved to differentvertical positions on the Y-rail 114 via a Y-rail drive 116 to accesstape cartridges on different rows of the storage shelves 104-105. TheY-rail drive 116 may comprise a motor turning a lead screw. The verticalrail 114 and gripper assembly 112 may be transported horizontally alonga horizontal rail 118 (also referred to as an X-rail) by an X-rail drive120. The X-rail 118 is mounted between the storage shelves 104-105 toallow the accessor 108 to travel between the storage shelves 104-105.The gripper assembly 112 may rotate approximately 180 degrees via arotational drive 122 to access the storage shelves 104 on the frontwall, and the storage shelves 105 and tape drives 106 on the rear wall.

FIG. 2 illustrates an X-rail drive 120 for accessor 108 in the priorart. The other elements of accessor 108 are not shown, but would beconnected to X-rail drive 120 and would extend upward out of the top ofpage in FIG. 2. X-rail drive 120 includes a drive motor 202 connected toa drive pulley 204 via a belt 206. Drive pulley 204 is connected to apinion (not visible) by a shaft. The pinion engages a rack 210 of X-rail118. When drive motor 202 turns drive pulley 204, drive pulley 204 turnsthe pinion on rack 210 to move accessor 108 in the X-direction. Guiderollers 216 for X-rail drive 120 engage a guide rod 218 of X-rail 118 toguide accessor 108 when moving in the X-direction.

FIG. 3 illustrates a tape library 300 comprising multiple storage frames102 in the prior art. In order to increase the size of tape library 100,additional storage frames 102 are bolted onto the original storage frame102 (see FIG. 1) to form a linear chain of storage frames 102. Tapelibrary 300 formed of multiple storage frames 102 may include at leasttwo accessors.

FIG. 4 is a top view of tape library 300 in the prior art. In FIG. 4,the storage frames 102 are connected side-by-side to form a linearchain. The storage shelves 104-105 are aligned respectively to form tworows of storage shelves. The X-rails 118 for each storage frame 102 areconnected to one another so that the accessors (ACC) 108 (shown in FIG.4 as boxes for illustrative purposes) may share a common X-rail 118 andaccess tape cartridges in any of the storage frames 102.

One problem with the tape library 300 shown in FIGS. 3-4 is that thetape library 300 can only grow in one linear direction. The tape library300 thus has a long, narrow footprint that may not work for the room orbuilding housing the tape library 100. One major limitation in how atape library can grow is the X-rail 118 and the X-rail drive 120 of theaccessor 108. Due to the configuration of the X-rail drive 120, theX-rail 118 needs to be a continuous rail and cannot branch off atangles, such as right angles. If an X-rail 118 were built with an angle,the X-rail drive 120 would not be able to negotiate the angle.Therefore, if the X-rail 118 needs to be continuous, then the shape ofthe tape library 300 is limited to a straight line, or a curved orcircular configuration.

Another problem with the configuration of tape library 300 is that amaximum number of two accessors 108 can be used. If either accessor 108in FIG. 4 becomes disabled, the other accessor can push the disabledaccessor to one end of tape library 300 and out of the way. If a thirdaccessor were to be added to tape library 300 and the middle accessorbecame disabled, then the disabled accessor would hinder access tocertain storage shelves 104-105.

FIG. 5 is a top view of a single U-shaped storage frame 502 in the priorart. The U-shaped storage frame 502 includes outside storage shelves 504and inside storage shelves 505. An accessor 508 is operable to accesstape cartridges in storage shelves 504-505 by traveling along U-shapedrail 510. By using a U-shaped storage frame 502, the tape library cangrow in two dimensions.

FIG. 6 is a top view of a tape library 600 using multiple U-shapedstorage frames 502 in the prior art. To expand in one direction, otherstorage frames 602 may be bolted onto the legs of the U-shaped storageframe 502 to expand the tape library 600. The accessor 508 for U-shapedstorage frame 502 can access the other storage frames 602 bolted ontothe end of storage frame 502. To expand in another direction, multipleU-shaped storage frames 502 can be connected side-by-side with apass-through mechanism 610.

There are multiple problems with this configuration. The U-shapedstorage frame 502 has a large radius in the corners of the “U” whichallows the accessor 508 to turn. However, the large curved areas make aregion where tape cartridge density is sacrificed as rectangular tapecartridges cannot be most efficiently packed around a curve. Anotherproblem is that the accessors 508 in one U-shaped storage frame 502cannot access tape cartridges stored in another U-shaped storage frame502, except through a pass-through mechanism 610. It is inefficient touse the pass-through mechanism 610 to pass tape cartridges from oneU-shaped storage frame 502 to another, as several different accessors508 need to handle the tape cartridges.

It would therefore be desirable to design a tape library, or other typesof data storage libraries, that have angled rails and accessors that cannegotiate the angled rails.

SUMMARY OF THE SOLUTION

The present invention solves the above and other related problems with adata storage library that has angled rails and accessors that cannegotiate the angled rails. For the rails of the data storage library,the data storage library includes at least a first rail and a secondrail. The second rail is oriented at an angle with the first rail, suchas a right angle or any other desired angle. The first rail and thesecond rail each have a drive surface, which may be located on the sideof each rail. The drive surface of the first rail has a longitudinalaxis X, and the drive surface of the second rail has a correspondinglongitudinal axis Z. There also exists a perpendicular axis Y that iscommon to the rails and perpendicular to the longitudinal axis X and thelongitudinal axis Z.

In this embodiment, the longitudinal axis X of the drive surface of thefirst rail is at a different position on the perpendicular axis Y thanthe longitudinal axis Z of the drive surface of the second rail. Forinstance, if the rails were configured horizontally, then the drivesurface of the first rail may be described as having a vertical positionthat is higher on the perpendicular axis Y than the vertical position ofthe drive surface of the second rail. The data storage library mayinclude multiple other rails that have a similar configuration as thesecond rail in relation to the first rail.

For the accessor of the data storage library, the accessor includes abase, a drive system, and a switching mechanism. The drive systemincludes a drive motor or some other drive means that rotates orotherwise drives a drive mechanism. The drive system is moveable uponthe base. The switching mechanism is affixed to the drive system and thebase, and is configured to move the drive system along the perpendicularaxis Y.

In operation, movement of the accessor may be switched from the firstrail to the second rail (and vice-versa) responsive to the positioningof the drive mechanism on the perpendicular axis Y by the switchingmechanism. For instance, if the switching mechanism positions the drivemechanism at a first position on the Y axis adjacent to the drivesurface of the first rail, then the drive mechanism may engage the drivesurface of the first rail to move the accessor along the first rail. Ifthe accessor needs to move along the second rail, then the switchingmechanism switches the position of the drive mechanism to a secondposition on the Y axis that is adjacent to the drive surface of thesecond rail. The drive mechanism may then engage the drive surface ofthe second rail to move the accessor along the second rail. The movementof the drive mechanism between the drive surfaces of the railsadvantageously allows the accessor to turn on angles.

The angled rails and the accessors having the ability to turn angledcorners and switch rails provide many advantages. First, the datastorage library may grow in two dimensions, instead of just linearly asin the prior art. For instance, the data storage library may comprise alinear chain of storage frames, with branches of storage framesexpanding off of the linear chain. The data storage library having thistopology may better suit the rooms or buildings housing the data storagelibrary.

The capability of the accessors to switch rails also allows theaccessors to pass one another along the rails. Because the data storagelibrary has multiple branches in this topography, any accessor canadvantageously move itself out of the way in one of the branches toallow other accessors to access any cartridge in the data storagelibrary without a pass-through mechanism. Therefore, more than oneaccessor may be used in the data storage library.

The switching mechanism for switching rails is advantageouslyimplemented in the accessor. The switching mechanism is a possible pointof failure in a data storage library. If the switching mechanism in oneaccessor fails, that accessor can be pushed out of the way or removedfrom the data storage library. Other accessors would still be able tooperate within the data storage library. If the switching mechanism wasimplemented in the switching rails themselves, a failure in theswitching mechanism would affect all accessors of the data storagelibrary.

The density of the storage shelves in each storage frame may also beincreased. The capability of the accessors to switch rails also allowsthe storage slots of the storage shelves to be put very close together(back-to-back) where if a large turning radius was required, morespacing would be required. This advantageously allows the cartridgedensity of data storage library to increase.

The invention may include other exemplary embodiments described below.

DESCRIPTION OF THE DRAWINGS

The same reference number represents the same element on all drawings.

FIG. 1 illustrates a tape library comprising a single storage frame inthe prior art.

FIG. 2 illustrates an X-rail drive for an accessor in the prior art.

FIG. 3 illustrates a tape library comprising multiple storage frames inthe prior art.

FIG. 4 is a top view of the tape library of FIG. 3 in the prior art.

FIG. 5 is a top view of a U-shaped storage frame in the prior art.

FIG. 6 is a top view of a tape library using U-shaped storage frames inthe prior art.

FIG. 7 illustrates an accessor and rails of a data storage library in anexemplary embodiment of the invention.

FIGS. 8-11 illustrate another embodiment of an accessor and rails of adata storage library.

FIG. 12 is a top view of a data storage library implementing theaccessor and the rails as described in FIGS. 7-11.

FIGS. 13-14 illustrate an alternative configuration for the accessorwhere the guide wheels of the accessor move with the drive system in anexemplary embodiment of the invention.

FIGS. 15-16 illustrate an alternative configuration for the rails of adata storage library in another exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 7-16 and the following description depict specific exemplaryembodiments of the present invention to teach those skilled in the arthow to make and use the best mode of the invention. For the purpose ofthis teaching, some conventional aspects of the invention have beensimplified or omitted. Those skilled in the art will appreciatevariations from these embodiments that fall within the scope of thepresent invention. Those skilled in the art will appreciate that thefeatures described below can be combined in various ways to formmultiple variations of the present invention. As a result, the inventionis not limited to the specific embodiments described below, but only bythe claims and their equivalents.

FIG. 7 illustrates an accessor 701 and rails 711-712 of a data storagelibrary in an exemplary embodiment of the invention. Only the driveportion of the accessor 701 is shown for the sake of brevity. The otherelements of the accessor are not shown, but would be connected to base704 of drive system 120 and would extend upward in FIG. 7 as isillustrated by the dotted lines. The data storage library may be a tapelibrary as shown in FIG. 1, or may comprise any other type of datastorage library utilizing an accessor.

In FIG. 7, rail 712 is oriented at an angle with respect to rail 711.Rail 712 may be oriented at a right angle or any other desired angle,such as a forty-five degree angle, a sixty degree angle, etc. In someembodiments, rail 712 may intersect, abut, or adjoin rail 711 and may beaffixed to rail 711. In other embodiments, the two rails 711-712, havingthe orientation described above, are not affixed to one another and maynot even contact one another.

Rails 711-712 are not a single continuous rail, but two separate railsthat are oriented with respect to one another to form an angle. The topsurface of rail 711 and rail 712 may be co-planar. Rail 711 has a drivesurface 713 illustrated as being positioned on a side of rail 711 sothat the drive surface 713 is vertical. Rail 712 also has a drivesurface 714 illustrated as being positioned on a side of rail 712 sothat the drive surface 714 is vertical. A drive surface is definedherein as any surface capable of being engaged by a drive mechanism ofan accessor so that the drive mechanism may exert force on the drivesurface to provide motion to the accessor. Drive surfaces 713-714 maycomprise smooth metal surfaces, rubber surfaces, racks (for arack-and-pinion system), or any other desired surface.

Drive surface 713 has a longitudinal axis X. The longitudinal axis X isdefined by drive surface 713 where the drive surfaces 713-714 areadjacent to one another, as the longitudinal axis X of drive surface 713may change away from this adjacent area. Drive surface 714 has acorresponding longitudinal axis Z. The longitudinal axis Z is defined bydrive surface 714 where the drive surfaces 713-714 are adjacent to oneanother, as the longitudinal axis Z of drive surface 714 may change awayfrom this adjacent area. FIG. 7 also shows a perpendicular axis Y thatis common to rails 711-712 and perpendicular to longitudinal axis X andlongitudinal axis Z.

In this embodiment, the longitudinal axis X of drive surface 713 is at adifferent position on perpendicular axis Y than the longitudinal axis Zof drive surface 714. If rails 711-712 are configured horizontally, asshown in FIG. 7, then drive surface 713 may be described as having avertical position that is higher on the perpendicular axis Y than thevertical position of the drive surface 714. The vertical position ofdrive surface 713 is higher than drive surface 714 in FIG. 7, but drivesurface 713 may be lower than drive surface 714 in other embodiments.

The vertical positioning of the drive surfaces 713-714 on the rails711-712 is of particular importance at the point where the drivesurfaces 713-714 are adjacent to one another (i.e., the point where theaccessor 701 switches rails 711-712). Drive surface 713 is at adifferent vertical position than drive surface 714 at this point. Awayfrom this point, drive surface 713 may be at the same vertical positionas drive surface 714. An example of this is shown in FIG. 13.

The data storage library of this embodiment may include a plurality ofother rails (not shown) having a similar configuration as rail 712. Adata storage library having such a configuration is shown in FIG. 12.

In the embodiment shown in FIG. 7, the accessor 701 includes base 704,drive system 702, and switching mechanism 706. The drive system 702 maycomprise the same or similar elements as the X-rail drive 120 describedin FIG. 2. Drive system 702 includes a drive motor 708 or some otherdrive means. Drive system 702 also includes some type of drive mechanism709 that is rotated or otherwise driven by the drive motor 708. Drivemechanism 709 may comprise a wheel, a sprocket, a pinion, or any othertype of mechanism configured to exert force on a drive surface to impartmotion to accessor 701.

Drive system 702 is moveable upon base 704, which is up and down in FIG.7. Switching mechanism 706 is affixed to the drive system 702 and thebase 704, and is configured to move the drive system 702 along the Yaxis. Switching mechanism 706 may comprise a solenoid, an air orhydraulic cylinder, a motor, a linear actuator, or some other mechanism.In this embodiment, the whole drive system 702 is moved vertically byswitching mechanism 706 relative to base 704 and rails 711-712. In otherembodiments, individual members of the drive system 702 may be moved byswitching mechanism 706. For instance, drive mechanism 709 may be movedindependently of the other elements of drive system 702.

Responsive to the positioning of the drive mechanism 709 on the Y axisby the switching mechanism 706, movement of accessor 701 may be switchedfrom rail 711 to rail 712, and vice-versa. For instance, if switchingmechanism 706 positions drive mechanism 709 at a first position on the Yaxis adjacent to drive surface 713, then drive mechanism 709 may engagedrive surface 713 to move accessor 701 along rail 711. Because drivesurface 714 is at a lower vertical position than drive surface 713,drive surface 714 is out of the way of drive mechanism 709. The otherportions of rail 712 are also out of the way to allow accessor 701 tomove along rail 711 in the direction of the X axis.

If accessor 701 needs to move along rail 712, then accessor 701 travelsalong rail 711 to be adjacent to rail 712 and drive surface 714.Switching mechanism 706 then switches the position of drive mechanism709 to a second position on the Y axis that is adjacent to drive surface714. In FIG. 7, switching mechanism 706 would lower drive mechanism 709on the Y axis until drive mechanism 709 is adjacent to drive surface714. Drive mechanism 709 may then engage drive surface 714 to moveaccessor 701 along rail 712. The movement of drive mechanism 709 betweenthe drive surfaces 713-714 advantageously allows the accessor 701 toturn on angles, such as the right angle shown in FIG. 7.

FIGS. 8-11 illustrate another embodiment of an accessor 801 and rails811-812 of a data storage library. FIG. 8 illustrates an accessor 801and rails 811-812 of a data storage library in an exemplary embodimentof the invention. Only the drive portion of the accessor 801 is shownfor the sake of brevity. The data storage library may be a tape libraryas shown in FIG. 1, or may comprise any other type of data storagelibrary utilizing an accessor.

Rail 812 is oriented at an angle with respect to rail 811. Rail 812 maybe oriented at another angle with respect to rail 811 in otherembodiments. Rail 811 has a rack 813, illustrated as being positioned ona side of rail 811, and a guide slot 815. Rail 812 also has a rack 814,illustrated as being positioned on a side of rail 812, and a guide slot816. A rack comprises a bar having teeth on one face for gearing with apinion or worm gear to transform rotary motion to linear motion. A guideslot comprises any track or groove to guide an accessor when travelingalong a rail.

Rails 811-812 are oriented horizontally in FIG. 8. Rack 813 of rail 811has a longitudinal axis X. Rack 814 of rail 812 has a correspondinglongitudinal axis Z. FIG. 8 also shows a perpendicular axis Y that iscommon to rails 811-812 and perpendicular to longitudinal axis X andlongitudinal axis Z.

In this embodiment, the longitudinal axis X of rack 813 is at adifferent vertical position on perpendicular axis Y than thelongitudinal axis Z of rack 814. More particularly, the verticalposition of rack 813 is higher on the perpendicular axis Y than thevertical position of the rack 814. With the different verticalpositioning of racks 813-814, a pinion 809 of the drive system 802engages only one of the racks 813-814 at a time.

In the embodiment shown in FIG. 8, the accessor 801 includes base 804,drive system 802, switching mechanism 806, and guide wheels 821-822.Drive system 802 includes a drive motor 805 connected to a drive pulley807 by a belt 808. The drive pulley 807 connects to pinion 809 by ashaft. The drive system 802 is moveable upon base 804 along the Y axisrelative to base 804 and rails 811-812. Switching mechanism 806 isaffixed to the drive system 802 and base 804, and is configured to movethe drive system 802 along the Y axis. Switching mechanism 806 maycomprise a solenoid, an air or hydraulic cylinder, a motor, a linearactuator, or some other mechanism.

Guide wheel 821 engages guide slot 815 of rail 811. Guide wheel 821guides accessor 801 when moving along rail 811. Guide wheel 822 is notengaging a guide slot in FIG. 8 and is used to engage guide slot 816 ofrail 812 when accessor 801 switches rails. The guide wheels 821-822 andguide slots 815-816 are just one example, and the guide wheels 821-822and guides slots 815-816 may have many desired configurations.

The configuration of accessor 801 and rails 811-812 allows accessor 801to switch between rails 811-812 and turn 90° corners. Assume thatswitching mechanism 806 positions pinion 809 at a first position on theY axis to engage rack 813. Drive motor 805 may then turn pinion 809 onrack 813 to move accessor 801 along rail 811. Because rack 814 is at alower vertical position than rack 813, rack 814 is out of the way ofpinion 809 when accessor 801 is moving along rail 811. The otherportions of rail 812 are also out of the way of rack 813 to allowaccessor 801 to move along rail 811 in the direction of the X axis.

FIG. 9 illustrates another view of accessor 801 and rails 811-812. Asshown in FIG. 9, guide slots 815-816 include openings 902. The openings902 allow the guide wheels 821-822 to disengage from guide slot 815 andengage guide slot 816 when accessor 801 is switching rails. Forinstance, guide wheel 822 will enter and engage guide slot 816 throughone of the openings 902, while guide wheel 821 will disengage guide slot815 through one of the openings 902.

FIG. 10 illustrates another view of accessor 801 and rails 811-812. Whenat this position on rail 811, accessor 801 may switch rails. In FIG. 10,accessor 801 is moved along rail 811 until the pinion 809 is adjacent torack 814 and guide wheels 821-822 are adjacent to the openings 902 inthe guide slots 815-816. Switching mechanism 806 may then lower thepinion 809 to a second position on the Y axis to engage rack 814 insteadof rack 813, as is illustrated by the arrow. FIG. 11 illustrates thepinion 809 engaging rack 814. With the pinion 809 positioned as in FIG.11, drive motor 805 may then turn pinion 809 on rack 814 to moveaccessor 801 along rail 812.

FIG. 12 illustrates a data storage library 1200 implementing accessorsand rails as described in FIGS. 7-11. The view in FIG. 12 is a top viewof data storage library 1202. Data storage library 1202 includes aplurality of rails 1211-1215 located between a plurality of storageshelves 1221-1228. Rail 1211 spans the width of the data storage library1202 to form a first linear row 1251. Rails 1212-1215 each abut rail1211 at right angles on different points along rail 1211. Rails1212-1215 thus extend out at substantially 90° angles to form acomb-like structure for rails 1211-1215. Storage shelves 1221-1228 arelocated on either side of rails 1212-1215 to form the branches 1252-1255of the data storage library 1202.

Data storage library 1202 includes a plurality of accessors 1241-1245that are configured to travel along the rails 1211-1215 to accesscartridges stored on the storage shelves 1221-1228. The cartridges maycomprise tape cartridges or another other type of storage media. Theaccessors 1241-1245 transport the cartridges between the storage shelves1221-1228 and the media drives 1230 of the data storage system 1202.

With the configuration of the rails and the accessors described in FIGS.7-11, the accessors 1241-1245 in FIG. 12 are able to turn angled cornersand switch rails 1211-1215. The capability of the accessors 1241-1245 toswitch rails 1211-1215 allows for multiple accessors 1241-1245 that canpass one another along the rails 1211-1215. Because data storage library1202 has multiple branches 1252-1255, any accessor 1241-1245 canadvantageously move itself out of the way in one of the branches1252-1255 to allow other accessors to access any cartridge in the datastorage library 1202 without a pass-through mechanism. The switchingmechanism for switching rails is also advantageously implemented inaccessors 1241-1245 to avoid a single point of failure. The capabilityof the accessors 1241-1245 to switch rails 1211-1215 also allows thedata storage library 1202 to advantageously grow in two dimensions. InFIG. 12, the branches 1252-1255 of the data storage library 1202 can beexpanded downward in FIG. 12. The branches 1252-1255 can be expandedindependently to different lengths to allow more flexibility in datastorage library 1202. The data storage library 1202 may also expandside-by-side by adding more storage frames to the left or right in FIG.12 along linear row 1251. The capability of the accessors 1241-1245 toswitch rails 1211-1215 also allows the storage slots of the storageshelves 1221-1228 to be put very close together (back-to-back) where ifa large turning radius was required, more spacing would be required.This advantageously allows the cartridge density of data storage libraryto increase.

FIGS. 13-14 illustrate an alternative configuration for the accessorwhere the guide wheels of the accessor move with the drive system in anexemplary embodiment of the invention. In FIG. 13, rail 1311 includes arack 1313 and two guide slots 1315-1316. Similarly, rail 1312 includes arack 1314 and two guide slots 1317-1318. In this embodiment, switchingmechanism 1306 moves guide wheels 1322 along with pinion 1309 and otherelements of a drive system (not shown). Switching mechanism 1306 raisespinion 1309 to engage rack 1313 of rail 1311 and at the same time raisesguide wheels 1321-1322 to engage guide slots 1315-1316. The accessor maythen move along rail 1311. To switch the accessor to rail 1312,switching mechanism 1306 lowers pinion 1309 to engage rack 1314 of rail1312 and at the same time lowers guide wheels 1321-1322 to engage guideslots 1317-1318. The accessor may then move along rail 1312. FIG. 14illustrates a top view of the rails 1311-1312.

FIGS. 15-16 illustrate an alternative configuration for the rails1511-1512 of a data storage library in another exemplary embodiment ofthe invention. In FIG. 15, rail 1511 includes a rack 1513 and two guideslots 1515-1516. Similarly, rail 1512 includes a rack 1514 and two guideslots 1517-1518. In this embodiment, rail 1512 also includes a curvedsection 1520 to provide a more gradual turn along the angle between rail1511 and rail 1512. The guide slots 1515-1518 include similar curvedsections 1521-1522.

At the point where an accessor would switch from rail 1511 to rail 1512,rack 1513 is at a higher vertical plane than rack 1514. The curvedsection 1520 is on the same plane as rack 1514. The drive mechanism ofthe accessor may be moved vertically any time while engaging rack 1513,even while moving. If the drive mechanism is in a lowered position whenapproaching the curved section 1520, then the drive mechanism willfollow the curved section 1520 and turn the accessor onto rail 1512. Ifthe drive mechanism is in a raised position when approaching the curvedsection 1520, then the drive mechanism will pass the curved section 1520and stay on rail 1511. Switching of rails 1511-1512 may thus beperformed more quickly. FIG. 16 illustrates a top view of the rails1511-1512.

The above description is of data storage libraries. The inventiondescribed herein applies equally to any robotic system that uses arobotic device to move along rails. A data storage library is just oneembodiment of a robotic system, and an accessor of a data storagelibrary is just one embodiment of a robotic device in a robotic system.

1-12. (canceled)
 13. A data storage library, comprising: a firsthorizontal rail having a first drive surface; a second horizontal railhaving a second drive surface and oriented at an angle with the firsthorizontal rail; wherein the vertical positioning of the first drivesurface on the first horizontal rail is higher than the verticalpositioning of the second drive surface on the second horizontal rail;and an accessor comprising: a drive mechanism configured to engage thefirst drive surface of the first horizontal rail to move the accessoralong the first horizontal rail, and to engage the second drive surfaceof the second horizontal rail to move the accessor along the secondhorizontal rail; a switching mechanism configured to move the drivemechanism vertically, wherein the switching mechanism positions thedrive mechanism at a first vertical position to engage the drivemechanism with the first drive surface of the first horizontal rail, andpositions the drive mechanism at a second vertical position to engagethe drive mechanism with the second drive surface of the secondhorizontal rail; and a guide wheel configured to engage a guide slot onthe first horizontal rail to guide the accessor when moving along thefirst horizontal rail, and engage a guide slot on the second horizontalrail to guide the accessor when moving along the second horizontal rail.14. The data storage library of claim 13 wherein the guide slot on thefirst horizontal rail includes an opening to allow the guide wheel todisengage from the guide slot on the first horizontal rail and engagethe guide slot on the second horizontal rail. 15-30. (canceled)